The present invention relates to an image pickup apparatus and, more particularly, to an image pickup apparatus such as a video camera which has a diaphragm mechanism for properly adjusting the amount of light incident on an image pickup element.
In an image pickup apparatus such as a video camera which has an image pickup element therein, it is quite common to use a mechanical diaphragm mechanism for adjusting the amount of light incident on the image pickup element through a lens in accordance with the brightness of an object to be photographed. Since the dynamic range of the image pickup element of a video camera is relatively narrow, a diaphragm mechanism must inevitably be used (especially in a video camera) to control the amount of light so as to obtain good image quality.
The diaphragm is closed to a suitable extent to achieve a proper exposure when an object to be photographed is bright (i.e., the illuminance of the object is high). And, when the object is dark (i.e., the illuminance of the object is low), the diaphragm is opened wide. Therefore, when the object illuminance is too high, for example, when the user wishes to take a picture of a snow scene in sunshine, the diaphragm mechanism of the conventional video camera simply closes the diaphragm. As a result, the diaphragm of the diaphragm mechanism is closed like a pinhole, and an undesirable "pinhole effect" occurs. When the diaphragm is fully closed like a pinhole, the depth of field is too deep, so that it is very difficult to properly focus the lens on the object using an electronic viewfinder of the video camera. Even if the distance between the video camera and the object does not change, when the illuminance of an object becomes low, proper focusing cannot be performed when focusing is adjusted for the dark object. Especially, when the user wishes to continuously take pictures of a moving object, the pictures of the moving object may be out of focus when the object illuminance is abruptly decreased, thus degrading the image quality of the object.
Furthermore, according to the conventional image pickup apparatus which has a diaphragm for simply changing the numerical aperture in accordance with the object illuminance, when the user takes a very bright scene (e.g., BV=12), scratches and dust on the surface of the image pickup element or the lens, and/or in the optical path are undesirably photographed due to the "pinhole effect" caused by excessive closing of the diaphragm. In this condition, the gain (transmissivity characteristics) of the diaphragm mechanism with respect to a change in the amount of image light passing therethrough is undesirably increased to a much greater extent than it is in any other condition. Therefore, in the above condition, the control circuit system of the image pickup apparatus becomes unstable, thus resulting in the "hunting" phenomenon.